1. Field of the Invention
This invention pertains to hollow core photonic band gap chalcogenide fibers and to the method of making same.
2. Description of Related Art
Practically every molecular species possesses a characteristic vibrational spectrum in the infrared wavelength region between about 2 and 12 μm. Consequently, chemicals can be clearly and definitely identified based on their unique infrared absorption spectra. In fact, this is called the “fingerprint region.” Detection and identification can be performed on liquids, gases and solids using traditional infrared (IR) spectrometers and Fourier Transform Infrared spectrometers. The chemicals can be detected using various modes including absorption and reflectance spectroscopy.
Fiber optic based sensors have been developed using chalcogenide glass fibers. Chalcogenide fibers transmit in the fingerprint region and can be coupled to an IR spectrometer. In this case, the evanescent field propagating outside the core can be accessed by removal of the cladding via etching or by reducing the diameter of the fiber by stretching. The evanescent field on the outside surface of the fiber, can interact with the surrounding analyte and provide definitive chemical identification even in mixtures. The advantage of the fiber approach is that the sensing can now be done remotely but one drawback of the evanescent spectroscopy is that the technique has limited sensitivity since only a small fraction, i.e., a few percent, of the light propagates in the evanescent field. Furthermore, the field decays exponentially from the surface of the fiber and so only analyte at the surface will interact with the evanescent field. Therefore, while IR spectroscopy is a very powerful tool for definitive chemical identification, there is a need to improve the sensitivity so that it will find widespread use.
Hollow core silica fibers with a structured region providing a photonic band gap have been known since about 1999. Photonic band gap fibers have been recently demonstrated using silica glass with a minimum loss of 13 dB/km at 1.5 μm. It would be obvious use hollow core photonic band gap silica fiber to transmit wavelengths longer than 1.5 μm since it was believed that the light was guided in the hollow core and therefore 2-12 μm light can be used to perform fingerprint IR spectroscopy. However, it turns out that hollow core PBG silica fibers do not transmit well beyond about 2 μm since several percent of the light propagates in the microstructured region and this light is highly attenuated through multiphonon absorption in silica. Hence, it is not possible to use silica PBG fibers to perform fingerprint spectroscopy in the IR beyond a wavelength of about 2 μm.